Refrigeration Pump Research Articles

Analysis and Optimization of Exergy Flows inside a Transcritical CO2 Ejector for Refrigeration, Air Conditioning and Heat Pump Cycles

In this study, the exergy analysis of a CO2 (R744) two-phase ejector was performed using a 1D model for both single and double choking conditions. The impact of the back pressure on the exergy destruction and exergy efficiencies was presented to evaluate the exergy performance under different working conditions. The results of two exergy performance criteria (transiting exergy efficiency and Grassmann exergy efficiency) were compared for three modes of an ejector functioning: Double choking, single choking and at the critical point. The behavior of three thermodynamic metrics: Exergy produced, exergy consumed and exergy destruction were evaluated. An important result concerning the ejector’s design was the presence of a maximum value of transiting exergy efficiency around the critical point. The impact of the gas cooler and evaporator pressure variations on the different types of exergy, the irreversibilities and the ejector global performance were investigated for a transcritical CO2 ejector system. It was also shown that the transiting exergy flow had an important effect on the exergy analysis of the system and the Grassmann exergy efficiency was not an appropriate criterion to evaluate a transcritical CO2 ejector performance.

Current Advances in Ejector Modeling, Experimentation and Applications for Refrigeration and Heat Pumps. Part 1: Single-Phase Ejectors

Ejectors used in refrigeration systems as entrainment and compression components or expanders, alone or in combination with other equipment devices, have gained renewed interest from the scientific community as a means of low temperature heat recovery and more efficient energy use. This paper summarizes the main findings and trends, in the area of heat-driven ejectors and ejector-based machines, using low boiling point working fluids, which were reported in the literature for a number of promising applications. An overall view of such systems is provided by discussing the ejector physics principles, as well as a review of the main developments in ejectors over the last few years. Recent achievements on thermally activated ejectors for single-phase compressible fluids are the main focus in this part of the review. Aspects related to their design, operation, theoretical and experimental approaches employed, analysis of the complex interacting phenomena taking place within the device, and performance are highlighted. Conventional and improved ejector refrigeration cycles are discussed. Some cycles of interest employing ejectors alone or boosted combinations are presented and their potential applications are indicated.

Current Advances in Ejector Modeling, Experimentation and Applications for Refrigeration and Heat Pumps. Part 2: Two-Phase Ejectors

Two-phase ejectors play a major role as refrigerant expansion devices in vapor compression systems and can find potential applications in many other industrial processes. As a result, they have become a focus of attention for the last few decades from the scientific community, not only for the expansion work recovery in a wide range of refrigeration and heat pump cycles but also in industrial processes as entrainment and mixing enhancement agents. This review provides relevant findings and trends, characterizing the design, operation and performance of the two-phase ejector as a component. Effects of geometry, operating conditions and the main developments in terms of theoretical and experimental approaches, rating methods and applications are discussed in detail. Ejector expansion refrigeration cycles (EERC) as well as the related theoretical and experimental research are reported. New and other relevant cycle combinations proposed in the recent literature are organized under theoretical and experimental headings by refrigerant types and/or by chronology whenever appropriate and systematically commented. This review brings out the fact that theoretical ejector and cycle studies outnumber experimental investigations and data generation. More emerging numerical studies of two-phase ejectors are a positive step, which has to be further supported by more validation work.

Experimental and simulation analysis of the combined adsorption system driven by 80–140 °C heat source

The shortage of fossil fuel, greenhouse gases emission, air pollution and global warming have attracted more and more researchers’ attention on the adsorption technology, which utilizes low temperature thermal heat of solar thermal energy and waste heat. Moreover, the usage of natural materials as refrigerant such as water and methanol adds to its advantage. However, the low coefficient of performance and specific cooling power limits its further application. This paper proposes a novel combined adsorption system, which can provide the functions of air conditioning/refrigeration/heat pump heating and ammonia/organic expanding power generation. In addition, the consolidated advanced adsorbent is used in this system, the effect of which is investigated. The author also analyzed the most suitable organic for organic Rankine cycle in this system. The results reveal that the organic of N-Butane has the highest potential exergy and the most appropriate work pressure. The cooling exergy efficiency in the working modes of air conditioning, refrigeration and heat pump heating are about 0.18, 0.13 and 0.24, respectively. The simulation results indicate that, compared to the system with only cooling effect, the total exergy efficiency of the combined system can be improved from 0.15 to 0.31, because of the additional expanding power generation effect.

Constructal design for disc-shaped heat exchanger with maximum thermal efficiency

Constructal design of a disc-shaped heat exchanger (DSHE) is implemented in this paper. Maximum thermal efficiency and total tube volume are taken as the optimization objective and constraint, respectively. Optimal tube lengths of the DSHE are obtained, and the effect of mass flow rate on its optimal construct is analyzed. The results show that for the first order DSHE, its thermal efficiency has the maximum and the elemental tube angle has the corresponding optimal value. For the higher order DSHEs, the optimal constructs of the DSHEs are not varied when their mass flow rates are small and large enough, and the DSHE structure is simplified into the structure of the last order DSHE when the mass flow rate is large enough. For the same tube volume and area of the DSHEs with 4 central tubes and a large mass flow rate, the thermal efficiency of the third order DSHE is increased by 7.33% than that of the first order DSHE. Therefore, the performance of the DSHE is increased when its structure becomes complex in this case. The obtained results can offer some instructions for the optimal designs of various thermal systems, such as power, refrigeration and heat pump plants.

Technical Assessment of a Hybrid Solar-geothermal System Including Adsorption Refrigerator for Energy Supply to Restaurants in Smart City

CO2 emissions from restaurants have increased over the past five years in Japan. As a result, countermeasures such as hybrid heat source systems that use renewable energy have gained increasing attention. In this paper, we assessed the efficiency of a hybrid solar-geothermal heat system including an adsorption refrigerator during its use in providing energy for air conditioning and hot water. We presented an energy prediction formula based on measured data on solar heat collection, cooling, and heating load. We also documented our evaluation of the performance of each component of the solar-geothermal heat system, i.e. the solar collector, adsorption refrigerator, and geothermal heat pump. And finally, we presented a case study on the use of this system in restaurants in a smart energy town in Japan. On solar collector, this study found that the heat collection efficiency during sunny and mostly sunny conditions were 28%. This percentage dropped to 23% for cloudy conditions and 11% for rainy conditions. On adsorption refrigerator, the Coefficient of Performance (COP) in summer was found to be lower than in autumn. On geothermal heat pump, the COP was found to be 4.5 during the cooling period and 4.3 during the heating period. Overall, the highest values for both energy consumption and energy utilization occurred in August, and the utilization efficiency was lower in winter than in summer. Moreover, the average annual energy utilization efficiency of the total system was 59.02%, while the Annual Performance Factor (APF) was 2.082. This study contributes to decision making and academic researches regarding the planning and implementation of energy-efficient cooling and heating system.

Molecular simulation and experimental investigation for thermodynamic properties of new refrigerant NBY-1 for high temperature heat pump

This paper presents the differences in thermodynamic properties of refrigerants in different spatial conformation, showing the superiority of molecular simulation in predicting thermodynamic properties of fluids. And molecular simulation was modified to improve prediction accuracy of thermodynamic properties of fluids at high temperature and pressure. Meanwhile, a new high temperature heat pump refrigerant mixture named NBY-1 was proposed, and its thermodynamic properties were estimated with the modified molecular simulation. Then the experiment investigation was carried out and showed the good cycle performance of NBY-1 as the refrigerant. The experimental results indicated that high temperature heat pump using NBY-1 could supply high temperature water of 130 °C with the highest coefficient of performance of 2.74 and the condensing pressure of 2.55 MPa, when the temperature lift was 50 °C. Moreover, the experiment also demonstrated the high accuracy with molecular simulation in predicting thermodynamic properties of NBY-1 in the two phase region, and the average relative deviation of condensing pressure between predicted results and experimental results was 2.59% with the highest value of 3.89%.

Performance Analysis of Refrigerants R1234yf, R1234ze and R134a in Ejector-Based Refrigeration Cycle

Performance enhancement of refrigeration and heat pump systems by cycle modification is an emerging research topic now-a-days to reduce the electricity consumption leading to mitigate the problems related to the environmental pollution by utility power plants. Due to no moving parts, low cost, simple structure and low maintenance requirements, the use of two-phase ejector has become a promising cycle modification recently. Use of ejector as an expansion device by replacing the throttle valve in the vapor compression refrigeration cycle seems to be one of the efficient ways to reduce the throttling losses or the expansion irreversibility in the refrigeration/heat pump cycle. Ejector also reduces the compressor work by raising the suction pressure to a level higher than that in the evaporator leading to the improvement of COP. The present work aims to evaluate the performance of an ejector based vapor compression refrigeration cycle under a wide range of operating conditions. Two newly proposed refrigerants i.e., R1234yf and R1234ze, and commonly used refrigerant R134a are considered for simulation and a comparative study has been carried out. A numerical model is developed and a parametric study of important parameters such as entrainment ratio, high side pressure (condenser pressure) and evaporator temperature are analyzed for the improvement of COP of the system. Results show that the COP of the R1234ze is highest compared to R1234yf and R134a for the given evaporating and condensing temperature.

Experiment research of solar PV/T cogeneration system on the building façade driven by a refrigerant pump

Solar energy technology applied to the building has shown a progressive trend in the architecture sector. Building integrated with the photovoltaic thermal system is called BIPV/T system, thus electrical energy is generated by photovoltaic modules and thermal energy is collected through working fluid. This paper presents an opaque ventilated PV/T façade system, which utilized the specific PV/T module as the external skin. A forced circulation with a refrigerant pump was adopted to cool the PV/T module and the coolant was R134a. The experimental research is carried out to evaluate the performance of the cogeneration system under outdoor climatic conditions of Dalian, including useful heat gain, the heating coefficient of performance, photovoltaic efficiency. In addition, thermal impacts on cavity air and solid wall are also analyzed. The results show that such system can reduce the heat flux of building envelope, and the system heating coefficient of performance can reach 3.1, the water temperature in thermal storage tank maximizes to 43.9 °C. The average photovoltaic efficiency is about 9% during the test period. Besides, the study shows that it is feasible to mount PV/T cogeneration system on the building façade. Furthermore, the system driven by a refrigerant pump operates stably and safely.

Research trend of compression type heat pump and natural refrigerant

Research trend of compression type heat pump and natural refrigerant

Pythagorean means and Carnot machines

Some interesting relations between Pythagorean means (arithmetic, geometric and harmonic means) and the coefficients of performance of reversible Carnot machines (heat engine, refrigerator and heat pump) are discussed.

Selection of the optimum prime mover and the working fluid in a regenerative organic rankine cycle

A regenerative organic Rankine cycle (RORC) is modeled and optimized for the use of waste heat recovery from a prime mover (PM). Three PMs including, a diesel engine, a gas engine, and a microturbine are selected in this study. Four refrigerants including isobutane, R123, R134a, and R245fa are selected. The nominal capacity of the PM, PM operating partial load, turbine inlet pressure, condenser pressure, refrigerant mass flow rate, pump efficiency, turbine efficiency, and regenerator effectiveness are considered as the decision variables. Then, the Genetic Algorithm is applied to maximize the thermal efficiency and minimize the total annual cost (TAC), simultaneously. The optimum results demonstrate that the best working fluid and the PM are, respectively, R123 and the diesel engine, which have a thermal efficiency of 0.50 and a TAC of $170,276/year. The optimum results are compared with each of the other studied cases. For example, the optimum result in the case of a diesel engine working with R123 shows a 2% and 2.52% improvement in the thermal efficiency and the TAC, respectively, in comparison to the case of a gas engine working with R123. Furthermore, a 26% and an 18.38% improvement in the thermal efficiency and the TAC are found when the best-studied cycle is compared with a microturbine and R123.

Modeling of two-phase transcritical CO2 ejectors for on-design and off-design conditions

CO2 two-phase ejectors are used as expansion work recovery components in refrigeration or heat pump systems. A new thermodynamic model is developed for both single choking and double choking conditions to design such ejectors and investigate their performances. This model, based on the homogeneous equilibrium model (HEM), solves the conservation equations using real fluid properties rather than ideal gas assumptions. The irreversibilities due to friction in the nozzles and the diffuser are accounted for by polytropic efficiencies. The performance of a two-phase ejector is predicted for a fixed ejector geometry and given inlet operating specifications at off-design conditions. The model can also evaluate the ejector dimensions for given inlet and outlet operating conditions (on-design). The results are compared and validated against published experimental data. The effects of the irreversibilities on the ejector dimensions are investigated in detail.

The performance of thermodynamic cycles based on the properties of working fluids

With the increase of global energy demand, themodynamic cycles, such as Rankine cycle, refrigeration or heat pump, have been widely employed to generate work or transfer heat. Since these cycles generally consist of a linked sequence of state points with physical properties variables, the physical properties of working fluid are essential to the cycle analysis. They directly determine the design of components, the cycle performance, the cycle stability and safety. However, in the thermodynamic analysis, the determination of cycle performance and the calculation of required physical properties are usually seperated. The thermodynamic peroperties of working fluids are often obtained from the complex experimental equations, so that the relationship between the cycle performance and the working fluids has not been established yet. What′s more, due to the fact that Carnot efficiency doesn′t contain detailed information on the properties of working fluids, a nature idea emerges how to derive the efficiency limit under the constraint of working fluids. Thus, in this work, the influences of thermodynamic properties on power cycles are investigated and the limiting efficiencys are proposed. In order to derive the cycle performances from the physical properties of working fluids, a general cubic equation of state is employed to obtain the residual properties, such as residual enthalpy, residual entropy and residual internal energy. Thereafter, on the basis of the properties of ideal gas, thermodynamic parameters of working fluids at any state are determined. According to the thermodynamic general relationships, the required heat is obtained for four classical processes, namely isochoric, isobaric, isothermal and isentropic processes. Based on the derived expressions, the output work and cycle efficiency are obtained for power cycles including Carnot cycle, Rankine cycle, Brayton cycle and Sterling cycle. The relationship between the cycle performance and the temperature, the properties of working fluids is developed. As a theoretical upper bound of cycle efficiency, Carnot efficiency is only determined by temperatures of heat source and sink. While the output work of Carnot cycle is a function of temperatures and the properties of working fluids. For other power cycles, it can be concluded that the thermodynamic performances are related with temperatures and working fluids, based on the derived expressions from cubic equation of state. Furthermore, compared with the Carnot efficiency under the same heat source and sink, the thermodynamic perfections of cycles are very low in practical engineering. Therefore, performance limits of thermodynamic cycles are investigated on the basis of the characteristics of working fluids in this paper. Limiting performance is derived from the equation of state and the temperature-entropy diagram of working fluids. For Rankine cycle and Brayton cycle, a maximum isobaric slope in the temperature-entropy diagram is employed to cut out the unexploited area from the enclosed area of Carnot cycle, so that the limiting work and efficiency can be obtained from the cycle areas. For Sterling cycle, when the used working fluid is ideal gas, the cycle efficiency is equal to Carnot efficiency. Although the proposed limiting performance can not be achieved by practical cycles, it can provide some theoretical guidance for the operating condition and the optimal design of thermodynamic cycles.

Constrained optimal high pressure equation of CO2 transcritical cycle

CO2 transcritical cycle has been widely applied in commercial refrigeration and heat pump water heating systems. Its cycle COP (coefficient of performance) heavily depends on the high pressure optimization and control. In theory, the optimal high pressure increases quickly with the increasing gas cooler outlet temperature or ambient temperature. However, due to reliability and cost concerns, there is an upper limit of the high pressure in compressor design and system operation. In this work, we proposed a constrained optimization method for getting constrained optimal high pressure equation of CO2 transcritical cycle. With the new method, the high pressure and its upper bound were simultaneously optimized at a given level of COP loss. Two specific approaches were developed to get the constrained optimal high pressure equation. Up to 1MPa high pressure reduction was achieved with about maximum 5% and average 1% COP losses.

Experimental investigation of an integrated cooling system driven by both liquid refrigerant pump and vapor compressor

The energy consumption of air conditioning system accounts for a significant proportion of the total energy consumed in data center. To reduce the energy consumption of the cooling system, an integrated system, which combines pumped refrigerant two-phase cycle (PRTC) with vapor compression cycle (VCC), is proposed in this paper. The impacts of input frequency and outdoor air temperature on the performances of PRTC, and that of the outdoor air temperature on the performances of VCC have been investigated. It was found that PRTC had higher EER than VCC, but its cooling capacity was limited by outdoor air temperature. In PRTC, the designed cooling capacity of 2500W could be obtained when the outdoor air temperature was −3.0°C. In addition, 80% and 50% of the designed cooling capacity can be obtained at ambient temperatures 3.2°C and 12.0°C, respectively. Besides, the feasibility of the system in different climate zones in China was demonstrated by analysis of the climate data in Harbin, Beijing, Wuhan, Guangzhou and Guiyang, respectively. Results indicated the proposed system had high feasibility in Harbin and Beijing. The results of this paper can provide valuable insights into the cooling system driven by both liquid refrigerant pump (LRP) and vapor compressor, targeting at reducing the cooling energy consumption and maximizing the economic efficiency of the cooling system in data center.

Off-Design Performances of Subcritical and Supercritical Organic Rankine Cycles in Geothermal Power Systems under an Optimal Control Strategy

The conditions of heat source and heat sink in a geothermal ORC system may frequently vary due to variations in geological conditions, ambient temperature and actual operation. In this study, an off-design performance prediction model for geothermal ORC systems is developed according to special designs of critical components, and an optimal control strategy which regards the turbine guide vane angle, the refrigerant pump rotational speed and the cooling water mass flow rate as control variables is proposed to maximize the net power output. Off-design performances of both subcritical and supercritical ORCs are analyzed. The results indicate that, under the optimal control strategy, the net power output of both ORCs increase with greater geothermal water mass flow rate, higher geothermal water inlet temperature and lower cooling water inlet temperature, which is mainly due to a greater working fluid mass flow rate, higher turbine inlet pressure and lower condensing pressure, respectively. The net power output of supercritical ORC is always greater than that of subcritical ORC within the range of this study, but the difference tends to decrease when supercritical ORC activates the geothermal water reinjection temperature restriction.

Absorption heat pump cycles with NH3 – ionic liquid working pairs

Ionic liquids (ILs), as novel absorbents, draw considerable attention for their potential roles in replacing water or LiBr aqueous solutions in conventional NH3/H2O or H2O/LiBr absorption refrigeration or heat pump cycles. In this paper, performances of 9 currently investigated NH3/ILs pairs are calculated and compared in terms of their applications in the single-effect absorption heat pumps (AHPs) for the floor heating of buildings. Among them, 4 pairs were reported for the first time in absorption cycles (including one which cannot operate for this specific heat pump application). The highest coefficient of performance (COP) was found for the working pair using [mmim][DMP] (1.79), and pairs with [emim][Tf2N] (1.74), [emim][SCN] (1.73) and [bmim][BF4] (1.70) also had better performances than that of the NH3/H2O pair (1.61). Furthermore, an optimization was conducted to investigate the performance of an ideal NH3/IL pair. The COP of the optimized mixture could reach 1.84. Discussions on the contributions of the generator heat and optimization results revealed some factors that could affect the performance. It could be concluded that the ideal IL candidates should show high absorption capabilities, large solubility difference between inlet and outlet of the generator, low molecular weights and low heat capacities. In addition, an economic analysis of the AHP using NH3/[emim][SCN] working pair with plate heat exchangers was carried out based on heat transfer calculations. The results indicated that the NH3/IL AHP is economically feasible. The efforts of heat transfer optimization in the solution heat exchanger and a low expense of ILs can help the IL-based AHP systems to become more promising.

Performance improvement of air source heat pump by using gas-injected rotary compressor

Rotary compressor is most widely used in small capacity refrigeration and heat pump systems. For the air source heat pump, the heating capacity and the COP will be obviously degraded when it is utilized in low temperature ambient. Gas injection is an effective method to enhance its performance under those situations, which has been well applied in air source heat pump with scroll compressor. However, the development of the gas injection technology in rotary compressor is relatively slow due to limited performance improvement. In this research, the essential reason constraining the improvement of the gas injection on the rotary compressor and its heat pump system is identified. Two new injection structures for rotary compressors has been put forward to overcome the drawback of traditional injection structures. Based on a verified numerical model, the thermodynamic performance of air source heat pumps with the new gas-injected rotary compressor are investigated. The results indicate that, compared to the air source heat pump with the traditional gas injected rotary compressor, the new injection structures both can enhance heating capacity and COP of the air source heat pump obviously.

Experimental results and design procedures for minichannel condensers and evaporators using propylene

Hydrocarbons (HCs) present favorable thermodynamic and transport properties that make them attractive for use in refrigeration, air conditioning and heat pump systems. The major issue related to HCs is their flammability and hence the realization of low charge heat transfer devices is a key factor for their market placement. Although flammable refrigerants call for charge minimization, propylene heat transfer data inside minichannels is new in the open literature.In the present paper, condensation heat transfer, flow boiling heat transfer and two-phase pressure drop measurements taken with propylene (R1270) inside a 0.96 mm diameter minichannel are presented and compared against correlations.A comparative analysis of the condensation performance of R1270, R290 and two HFCs is made with the objective of minimizing the exergy losses, due to the driving temperature difference between saturation and wall and to the frictional pressure drop. An estimation of the condenser refrigerant charge is also performed. The flow boiling heat transfer performance of R1270 is discussed in comparison to the one of R290.